This invention relates to finely divided normally solid, synthetic organic polymeric thermoplastic resins.
Thermoplastic polymers in powder or finely divided form have a wide variety of commercial applications, such as for example, the dry powders have been used to coat articles in dry form by dip coating in either static or fluidized beds, by electrostatic coating, spraying or dusting and flame spraying. The powders are used in dispersed form in suitable liquid carriers to apply coatings by roller coating, spray coating, and dip coating to a variety of substrates such as; glass, ceramics, metal, wood, cloth, paper, paperboard, and the like. The finely divided polymers have also been successfully employed in conventional powder molding techniques. The fine powders have also been applied as paper pulp additives, mold release agents, wax polish, paint compositions, binders for non woven fabrics and finishes for woven fabrics.
There are basically four types of processes employed in the prior art for preparing finely divided polymer particles, i.e., mechanical grinding, solvent precipitated, dispersion and spray atomization of solutions or slurries.
Generally mechanical grinding employs conventional equipment to yield particles of irregular shape and wide size variation of from about 75 to 300 microns. The powders produced by this method may not be suitable for applications where free flowing powders are required, since the irregular shapes may inhibit the flowability of these powders. The grinding of some polymer may be very costly because of the toughness of the resin even when cryogenically cooled.
The spray techniques are generally satisfactory for producing uniform non-agglomerated, spherical particles, however, very specialized equipment, usually nozzles operating under a limited range of conditions to prevent nozzle plugging are required. Substantial problems in spraying are the shearing of the polymer as it passes through the nozzle, premature precipitation of the polymer or rapid volatilization of solvent.
The dispersion method also is subject to high shear conditions. Frequently water is the dispersing medium and dispersing agents are used to facilitate the dispersion. Hence the powders produced by this technique generally incorporate some or all of the dispersing agent in the powder which can create undesirable changes in the original polymer properties, e.g., increased water sensitivity, loss of electrical insulating values, loss of adhesive capabilities, etc.
The final type of prior art process generally entails dissolving the polymer in a solvent, followed by precipitation of the polymer in finely divided form through addition of a nonsolvent, cooling or evaporation of the solvent or a combination of the above. Problems in this process have included difficulty in manipulating the solvents, solvent removal, particle agglomeration which requires a great deal of grinding, and particles having irregular somewhat rounded shapes.
Powders produced by grinding or emulsification of a polymer melt produces non-porous powder particles.
Coating of substantially non-porous substrates such as glass or metal has frequently been characterized by poor surface bonding or poor resistance of the bond to certain environments. Non-returnable glass bottles have been covered with polstyrene or polyethylene jackets or ionomer resin to reduce breakage and retain fragments if broken, however, these coatings are not intended to be permanently bonded. Even when a bond is obtained, it is usually the case that the resin or bond will not withstand the caustic rinses at 150.degree. to 160.degree. F. used on returnable bottles.
The objective of placing polymer coatings on returnable bottles is to reduce scratches and impact on the glass and hence increase the useful life of each bottle. The coating should also provide a safer bottle, reducing the likelihood of breakage, and in the event of breakage, contain the shattered glass or a portion thereof within the plastic skin.